Thermosetting resins used in casting or open and close mold applications are typically cured by a free radical polymerization process. Examples of such thermosetting resin include unsaturated polyester resins, vinyl ester resins and urethane(meth)acrylates. The backbone of these resins either contain ethylenically unsaturated groups such as fumarate or (meth)acrylate and are dissolved in a liquid copolymerisable monomer such as styrene, methyl methacrylate or vinyl toluene. Such resins are liquid under normal conditions but, when treated with a source of free radicals such as an organic peroxide initiator an in the presence of a promoter, will rapidly crosslink to form a hard thermoset crosslinked network. Such a process is used in the production of, for example, castings, coatings, adhesives and fiber reinforced articles.
Commercially available promoter systems for ambient cure thermosetting resins include accelerators or promoters used in conjunction with the initiator and include, for example, salts of metals chosen from among lithium, calcium, copper, vanadium, zirconium, titanium, zinc, iron, sodium, potassium, magnesium, manganese, barium and cobalt, in combination with one or more compounds of alkyl organic acids, halides, nitrates to form a coordination compound. Any skill in the art can choose a metal salt comprising a combination of transition metal salts. This means e.g. one kind of transition metal but different coordination ions or ligands; different transition metal ions and one kind of coordinating ions or ligands; and combinations of these. The accelerator may be added in several different manners. For example, the accelerator may be pre-mixed to form a metal salt complex prior to it being added to the resin composition. Another possibility is to add the individual components of the accelerator composition to the resin and form the metal complex in situ. The most preferred method will depend on the specific curing process being carried out. Examples of the metal salts are described for example in PCT Publication Nos. WO9012824A1 and WO03093384A1, and U.S. Pat. No. 8,039,559 (B2); the disclosures of which are incorporated herein by reference in their entirety.
The choice of the metal ion of metal salts depends upon several parameters, such as activity at ambient temperatures, possible coloring effects, toxicity, stability in the thermoset product, price, and the like. It should be taken into account that the activity of the metal ion also depends upon the kind of coordinating groups. Because of their good performance at ambient temperature, cobalt-containing accelerators are the most widely used copromoters. However, a disadvantage of cobalt is that cobalt carboxylates are suspect to high toxicity (carcinogenicity). Hence, there is an increasing demand in the thermosetting resin industry for promoters that can provide an appropriate curing without compromising performance of the resulting products.
Much attention has recently been given to thermosetting systems that can be cured via free radical polymerization together with a variety of accelerators. More in particular, accelerators that are free of any cobalt salts because cobalt carboxylates are suspect to high toxicity (carcinogenicity). Various patents describe promoter systems that do not include cobalt salts and that are able to cure thermosetting resins. Some of these promoter systems are described for example in U.S. Pat. No. 8,039,559 and PCT Publication Nos. WO2005047379A1 and WO2006131295A1.
WO2011083309A1 and WO2011124282A1 describe the preparation of accelerators based on iron/manganese complexes of tridentate, tetradentate, pentadentate, or hexadentate nitrogen donor ligands. These publications describe metal iron and manganes metal complexes able to cure unsaturated polyesters and vinyl esters and do not include any cobalt salts. The nitrogen ligands are made from structures which present some difficulty on the preparation of their chemical structures. Multiple steps are required in there preparation which make the products more expensive due to their fabrication complexity.
There remains the need to have metal complexes that do not include cobalt to cure thermosetting resin systems via free radical polymerization at room temperature or at moderate temperatures, with excellent processability, without compromising their mechanical properties. In addition, it would be advantageous to have a simple and affordable process that would yield products free of toxic or harmful components in the resulting thermosetting materials.